This invention relates to electrical storage cells, and, more particularly, to a corrosion-resistant nickel electrode used in such cells.
Rechargeable cells or batteries are electrochemical devices for storing and retaining an electrical charge and later delivering that charge as useful power. A familiar example of the rechargeable cell is the nickel-cadmium cell used in various portable electronic devices such as cameras and radios. Another type of cell having a greater storage capacity for its weight is the nickel-hydrogen cell. The nickel-hydrogen cell is used in spacecraft such as satellites to store power generated by solar cells when the spacecraft is in sunlight, and discharged to supply power when the spacecraft is in darkness.
These storage cells utilize a nickel positive electrode, a negative electrode, and an electrolyte as the basic charge-storing element. For example, the nickel-hydrogen cell includes a series of active plate sets, each plate set having a nickel/nickel oxide positive electrode (termed a "nickel electrode"), a hydrogen negative electrode, a separator between the electrodes, and an electrolyte such as a potassium hydroxide solution. A stack of the plate sets are packaged within a pressure vessel that contains the stack, the electrolyte, and the hydrogen gas evolved and consumed during the charge/discharge cycle of the cell. A nickel-hydrogen storage cell has an open circuit voltage of about 1.3 volts, and a number of the cells are usually connected in series to provide the voltage required by the systems of the spacecraft.
The nickel electrode is manufactured by impregnating a porous nickel substrate with nickel oxide. The nickel substrate is corroded in the aqueous impregnation solution, which is acidic. The corrosion reduces the utilization of the nickel oxide active material and the cyclic life of the nickel electrode and thence the life of the cell. Since a storage cell in a commercial satellite may be required to operate for periods of time of 15 years or more without any maintenance, reduction of the cyclic life due to corrosion of the storage cell is a serious concern.
There have been several approaches to reducing corrosion of the nickel substrate during manufacture of the nickel electrode. One technique is to use an alcohol-based impregnation bath, but this approach, used presently, may be unacceptable in the future in large-scale manufacturing operations due to environmental concerns. Another approach is to use an aqueous impregnation bath, but to inhibit corrosion of the nickel substrate of the nickel electrode by passivating the nickel substrate. The passivation is accomplished by dampening the substrate with water and heating it in air to a temperature of about 350.degree. C. A nickel oxide film forms on the surface of the nickel substrate, and the passivating film is somewhat effective in reducing corrosion.
However, these approaches have not been entirely successful, and there exists a need for an improved approach to preventing corrosion of the nickel substrate portion of the nickel electrode during impregnation of the nickel oxide into the porous nickel substrate and during service of the storage cell. The present invention fulfills this need, and further provides related advantages.